JP2010220031A - Transmission power control on which plurality of base stations cooperate, and antenna beam selective control method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maximize the number of users simultaneously connected in a cellular system. <P>SOLUTION: Assuming that each terminal has an SINR equivalent to a required transmission quality for all combinations of antenna beams of base stations, simultaneous equations having variables of antenna beam and transmission power selected for each base station are established to find a transmission power for each base station. When the solutions of the equations fail to meet restriction requirements of the base station power, the above operation is repeated by sequentially reducing the number of terminals to determine an antenna beam combination and a base station transmission power for the largest number of users simultaneously connected. In order to reduce the quantity of calculation, multiple antenna beams having strong signal intensities at the terminals may be extracted and the other antenna beams having weak signal intensities may be excluded from the search targets. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a cellular communication system that forms a service area by combining a plurality of radio zones (cells) formed by a plurality of base stations.

In the cellular communication system, the frequency used by each cell is reused between cells, and signals having the same frequency from other cells become interference.
FIG. 7 is a diagram for explaining a situation of interference from other cells in the downlink.
In this figure, BS is a base station and MS is a terminal. Here, it is assumed that adjacent cells use the same frequency (one-cell frequency repetition). As illustrated, the terminal MS _i belonging to the base station BS _i, so that the interference signal from the base station BS of another cell, such as a base station BS _k with the desired signal from the base station BS _i is received.

周辺からの干渉が大きくなればなるほど式（１）の分母が大きくなるため、ＳＩＮＲが低くなり、伝送品質が悪くなってしまう。
このように、セルラ通信システムでは、端末が基地局と通信を行う際、周辺基地局で送信される同一周波数の電波が干渉となり、端末において十分な伝送品質を得ることができず、システム容量（スループットやユーザの同時接続数）が制限されてしまうという問題がある。 SINR (signal to interference noise ratio) is used as an index representing transmission quality. If the SINR is high, the transmission quality is good, and if the SINR is low, it is bad.
SINR is expressed by the following equation.

The greater the interference from the periphery, the larger the denominator of equation (1), so the SINR is lowered and the transmission quality is degraded.
Thus, in a cellular communication system, when a terminal communicates with a base station, radio waves of the same frequency transmitted from neighboring base stations interfere with each other, and sufficient transmission quality cannot be obtained at the terminal. There is a problem that the throughput and the number of simultaneous connections of users) are limited.

Therefore, the present inventor calculates the interference that each base station gives to the terminals of the neighboring base stations in advance, and multi-base station cooperative transmission power that cooperates with the power control of the base station so that each terminal can obtain the required transmission quality. A control method is proposed (Japanese Patent Application Nos. 2008-223841, 2008-223842, 2008-223843, Non-Patent Document 1).
The proposed multiple base station cooperative transmission power control method will be described.
Assume 1-cell repetition at a frequency as shown in FIG. In each cell, only one terminal using the frequency can exist as a terminal capable of communication.
The transmission power of the base station BS _i p _i, the transmission power of the base station BS _k p _k, normalized received power of z _ii from the base station BS _i to the terminal MS _i belonging to the base station BS _i, the base station BS _k If the propagation loss from the terminal MS _i to the terminal MS _i is z _ik , the desired signal power is p _i z _ii and the interference signal power from the base station BS _k is p _k z _ik .
When the number of base stations is N (N is an integer of 2 or more) and the noise power at the terminal MS _i is n _i , the received SINR _{i at the} terminal MS _i satisfies the required transmission quality γ _{req, i} as follows: It is necessary to satisfy the conditions.

For all terminals that exist in all base stations, create a conditional expression that converts the inequality sign in equation (2) above to an equal sign, that is, the transmission quality of each terminal is equal to the required transmission quality. Then, simultaneous equations with the power of each base station as a variable are obtained. However, in the ceremony, it is necessary to estimate the standardized received power and noise level from each base station at each terminal.
By solving this simultaneous equation, it is possible to obtain the transmission power of each base station when the transmission quality at all terminals becomes the required transmission quality γ _{req, i} . However, the transmission power p _i (i = 1 to N) of each base station needs to satisfy 0 ≦ p _i ≦ P _limit where the maximum transmission power of the base station is P _limit (constraint conditions for base station power) . However, the power solution may be out of the range that the base station can output. For example, the solution may be a negative value.

If the solution of the simultaneous equations does not satisfy the constraint conditions, it is impossible for all terminals to communicate with the required quality. Therefore, the number of terminals that allow communication is reduced so as to satisfy the constraint conditions. Temporarily cut off the communication of several terminals, that is, cut off the transmission power of the base station to which the terminal belongs, thereby reducing the variables and formulating the simultaneous equations again so that the solution satisfies the constraints. Find the solution. Here, when reducing the number of terminals, an evaluation rule is to increase the number of terminals that can be connected simultaneously as much as possible.
The most effective method is to solve simultaneous equations for all combinations of terminals that allow communication, and to find a combination of terminals that satisfy the base station power constraint condition that maximizes the number of simultaneously connected terminals (total Hit method).
In addition, the round robin method can obtain accurate results by sequentially formulating simultaneous equations for all combinations that remove i terminals from N terminals, and can obtain accurate results. There is a problem that the amount of calculation increases exponentially as the number of stations increases.
Therefore, the present inventor has proposed in the above patent application a base station transmission power control method and apparatus in a cellular communication system that can greatly improve the system capacity with a small amount of calculation.

In addition, there are the following two methods as conventional base station power adjustment methods.
One is a method in which the same power is set and transmitted to all base stations, and the other is a base so that a required SNR (signal to noise power ratio) is achieved for terminals in each base station. This is a method of performing power control for each station. However, neither method considers the interference power from the neighboring base stations. Therefore, there is a possibility that SINR deteriorates due to the interference and the required transmission quality cannot be satisfied.
On the other hand, by introducing directivity to the antenna, it is possible to transmit radio waves in a concentrated manner toward a target terminal, and thus it is possible to reduce interference with terminals belonging to other cells. As a conventional technique of antenna beams, there is known a method of selecting a beam having the highest received power at a terminal using a multi-beam antenna capable of forming a plurality of beams having different main beam directions as a base station transmission antenna. Yes. Similarly, in this method, interference with other terminals is not considered and a high user connection rate cannot be obtained.

Hoshino Kenji, Nagashi Atsushi, Fujii Teruya, "A Study on Cooperative Transmission Power Control Method for Multiple Base Stations in Mobile Communications", 2008 IEICE Society Conference, B-5-51, p.364, September 2008

According to the proposed multiple base station cooperative transmission power control method, the transmission power of each base station is set so that the number of simultaneously connected terminals can be increased as much as possible while satisfying the constraints of the base station power. However, this is not the case when each base station uses a multi-beam antenna capable of forming a plurality of beams having different main beam directions as a transmission antenna.
In addition, there is a conventional technique for directing an antenna beam in various directions, and it is also known to perform antenna beam selection that selects one appropriate beam from among a plurality of antenna beams that a base station has. However, the beam with the highest signal power is usually selected for the terminal, and the method of selecting the antenna beam considering the influence on the terminal belonging to another base station has been Not proposed.

  Therefore, the present invention provides a multiple base station cooperative transmission power control and antenna beam selection control method capable of maximizing or improving a user simultaneous connection rate by performing transmission power cooperative control and antenna beam selection control in a plurality of base stations, and The object is to provide a device.

In order to achieve the above object, the multiple base station cooperative transmission power control and antenna beam selection control method of the present invention includes a transmission power in the plurality of base stations in a cellular communication system having a plurality of base stations and a plurality of terminals. A multiple base station cooperative transmission power control and antenna beam selection control method for controlling a beam direction of a transmission antenna, wherein the plurality of base stations are configured such that transmission quality of a terminal belonging to each of the plurality of base stations becomes a required transmission quality. A first step of formulating simultaneous equations with the beam direction and transmission power of the transmitting antenna in the station as variables, and a solution of the simultaneous equations, and the solution of the simultaneous equations satisfy the base station power constraints A second step of determining the solution as a beam direction and transmission power of a transmission antenna in the plurality of base stations, and the simultaneous equations A third step of moving to the first step by reducing terminals belonging to the selected base station of the plurality of base stations when the solution does not satisfy the base station power constraint condition; It is what has.
Further, in each base station of the plurality of base stations, the first step to the third step are executed for all combinations of the beam directions of the transmission antennas of the base stations of the plurality of base stations. Selecting the beam direction having the strongest signal strength received by a terminal belonging to the base station as the beam direction of the transmitting antenna of the base station, and performing the first to third steps, or In each base station of the base station, the first to third steps are executed for all combinations of a plurality of beam directions selected in descending order of the signal strength received by the terminals belonging to the base station. .
Further, the multiple base station cooperative transmission power control and antenna beam selection control apparatus according to the present invention provides the transmission power and the beam direction of the transmission antenna in the plurality of base stations in a cellular communication system having a plurality of base stations and a plurality of terminals. A multiple base station cooperative transmission power control and antenna beam selection control apparatus for controlling, wherein the transmission antenna beams in the plurality of base stations are set such that transmission quality of terminals belonging to the plurality of base stations becomes a required transmission quality. First, a simultaneous equation having a direction and a transmission power as a variable is formed, a first means for obtaining a solution of the simultaneous equation, and when the solution of the simultaneous equation satisfies a base station power constraint condition, A second means for determining a beam direction and transmission power of a transmission antenna in the plurality of base stations; And the third means for reducing the number of terminals belonging to the selected base station among the plurality of base stations and executing the processing of the first means when the constraint condition is not satisfied. is there.

  According to the multiple base station cooperative transmission power control and antenna beam selection control method and apparatus of the present invention as described above, an antenna suitable for the direction of the terminal in consideration of interference given to the terminals of the neighboring base stations between the base stations in advance. Select the beam and control the power of the base station so that the required received power can be obtained, and allow communication if all terminals cannot meet the required power and determine the base station power By reducing the number of terminals and determining the power for the remaining base stations, in cellular mobile communications, radio waves from the base stations are interfered by radio waves from neighboring base stations, and sufficient reception quality is obtained at the terminals. Therefore, it is possible to solve the problem that the number of simultaneous connections of users is limited, and to maximize or improve the simultaneous user connection rate.

It is a figure which shows the structure of the cellular communication system to which the multiple base station cooperation transmission power control and antenna beam selection control method of this invention are applied. It is a figure for demonstrating the mode of the interference from the other cell in a downlink when the directivity of a base station transmission antenna is considered. It is a figure which shows an example of the directivity pattern of a multi-beam antenna. It is a flowchart which shows the flow of a process of multiple base station cooperation transmission power control and antenna beam selection control of this invention. It is a figure for demonstrating embodiment which tried to reduce the amount of calculations by considering only the top k beam with strong receiving intensity of a terminal. It is a figure which shows the result of having evaluated the user simultaneous connection rate in the multiple base station cooperation transmission power control and antenna beam selection control method of this invention by computer simulation. It is a figure for demonstrating the mode of the interference from the other cell in a downlink.

FIG. 1 is a diagram illustrating a configuration of a cellular communication system to which a multi-base station cooperative transmission power control and antenna beam selection control method of the present invention is applied. In this figure, 1 ₁ to 1 _N are base stations (BS) (N is an integer of 2 or more), 2 ₁ to 2 _N are terminals (MS), and 3 is a control station that controls the plurality of base stations. Each base station 1 ₁ to 1 _N forms a cell, and each base station uses the same frequency. Each of the base stations 1 ₁ to 1 _N includes a multi-beam antenna capable of forming a plurality of beams having different main beam directions, and uses the beam determined by the method of the present invention among the plurality of beams. It communicates with the terminals 2 ₁ to 2 _N located in its own cell. In this figure, it is described that the terminals 2 ₁ to 2 _N exist in all the cells. However, when the terminal 2 does not exist in the cell, a plurality of terminals 2 exist in one cell. May be present. However, since the same frequency is used, only one terminal 2 can communicate with the base station using that frequency within one cell.
Control station 3, the connected to a plurality of base stations 1 ₁ to 1 _N, the beam of the transmitting antenna of each base station 1 ₁ to 1 on the basis of such on information obtained through the _N base stations 1 ₁ to 1 _N The direction and transmission power are determined, and the determined beam direction and transmission power of the transmission antenna are notified to each of the base stations 1 ₁ to 1 _N. The base stations 1 ₁ to 1 _N select a beam corresponding to the direction notified as the beam of the transmission antenna, and control the transmission power of the base station 1 ₁ to 1 _N to be the transmission power notified from the control station 3. In the present invention, a plurality of base stations cooperate in this way to perform transmission power control and antenna beam selection control.

FIG. 2 is a diagram for explaining the state of interference from other cells in the downlink when the directivity of the base station transmission antenna is taken into consideration.
As shown in this figure, when each base station uses a multi-beam antenna having a plurality of antenna beams, the directivity of the antenna is reduced. Therefore, compared to the case where an omni antenna is used, The influence on the terminal to which it belongs can be reduced. Furthermore, interference with terminals belonging to other stations can be reduced by selecting a beam having a desired signal strength for the terminals belonging to the own station while taking into consideration the influence on the terminals belonging to other base stations. It is possible to increase the number of terminals that can be connected simultaneously in the entire system.

Hereinafter, the multi-base station cooperative transmission power control and antenna beam selection control processing for determining the beam direction and transmission power of the transmission antennas of the base stations 1 ₁ to 1 _N executed in the control station 3 will be described.
In the multiple base station cooperative transmission power control and antenna beam selection control method of the present invention, in order to determine the optimal base station transmission antenna beam combination and base station transmission power that maximizes the simultaneous user connection rate, Simultaneous equations using the beam direction of the transmission antenna and the base station transmission power as variables are established, and the transmission power of each base station is determined so that the received SINR at the terminal satisfies the required transmission quality.

図３に、アンテナ素子の指向性パターン（Ｇ）の一例を示す。この図に示すマルチビームアンテナは３通りのアンテナビームを有し、各ビームの指向方向が１２０度ずつ異なっている。 The transmission power of the base station BS _k p _k, the base station BS _k is m _k-th antenna gain of the antenna beams from the base station BS _k in the case of selecting to terminal MS _i belonging to the base station BS _i G _ik (m _k ), where the channel gain is z _ik , the number of base stations is N, and the noise power at the terminal MS _i is n _i , the received SINR (γ _i ) at the terminal MS _i is the required transmission quality (γ _{req, i} ). The conditions for satisfying are as follows.

FIG. 3 shows an example of the directivity pattern (G) of the antenna element. The multi-beam antenna shown in this figure has three types of antenna beams, and the directivity directions of the beams are different by 120 degrees.

For all the terminals, when the equation in which the inequality sign in the above equation (3) is converted to the equal sign, that is, the equation in which the transmission quality of each terminal is equal to the required transmission quality is created and expanded, Simultaneous equations with the directivity (antenna beam selected at each base station) and the transmission power of the transmission antennas as variables are obtained. However, the control station 3 connected to all the base stations 11 ₁ to 1 _N to be coordinated has a propagation path (propagation loss) from each base station 1 ₁ to 1 _N to all the terminals 2 ₁ to 2 _N and each It is assumed that all the noise power n _i at the terminal is grasped as information. For example, each base station sequentially transmits a pilot signal while switching the beam, and each terminal measures the signal strength and noise of the received pilot signal and notifies the control station 3 via each base station. Has been made.

FIG. 4 is a flowchart showing the flow of multi-base station cooperative transmission power control and antenna beam selection control processing.
First, in order to formulate simultaneous equations based on the above equation (3), information on the propagation path from each base station to all terminals and information on noise in each terminal are acquired (step S1).
Next, one combination of antenna beams m _k used at each base station is selected (steps S2 and S3).
Next, proceeding to step S4, each base station when γ _i = γ _{req, i} is obtained for all terminals MS _i by solving simultaneous equations in which the inequality sign in equation (3) is replaced with an equal sign. Obtain the transmission power of. However, the maximum transmission power of the base station is P _limit , and 0 ≦ p _i ≦ P _limit is a limiting condition. If the solution does not satisfy this constraint condition, it is impossible for all terminals to perform communication with the required quality at the same time. Therefore, the number of terminals that permit communication is reduced so as to satisfy the condition. When the number of terminals is reduced, the transmission power from the base station to which the terminal belongs becomes 0, and the amount of interference with terminals belonging to other base stations decreases. When reducing the number of terminals, maximizing the number of terminals that can be connected simultaneously is used as an evaluation criterion, and the number of terminals that allow communication until the condition is satisfied is sequentially reduced.

  In addition, the method of terminal reduction is not asked. As a method of maximizing the connection rate, solve the simultaneous equations for all combinations of terminals that allow communication, and find the combination of terminals that satisfy the power constraint condition that maximizes the number of simultaneously connected terminals (Full search). However, in this method, the amount of calculation increases exponentially as the number of base stations to be cooperatively controlled increases. Therefore, select the terminal to be removed uniquely, solve the simultaneous equations each time one terminal is removed, determine whether the solution satisfies the power constraint condition, and if not, repeat the process of selecting the terminal to be removed again There is a method to reduce the amount of calculation. This method is described in the above-mentioned patent application by the present inventor. For example, assuming that all base stations are transmitting at the maximum power, the received SINR at each terminal is calculated and the value is calculated. There are a method of removing terminals in order from the lowest terminal (Lowest-SINR removal), a method of removing from terminals belonging to the base station having the largest interference to all terminals (Largest-IF removal), and the like. In the present invention, any method may be adopted to reduce the number of terminals.

The above transmission power allocation control is performed for all antenna beam combinations (steps S5 and S6). When the number of antenna beams of each base station is M and the number of base stations is N, there are a total of ^MN combinations of all antenna beams.
After performing transmission power allocation control for all combinations of antenna beams of each base station, a beam having the largest number of simultaneous user connections is set as a final selected beam (step S7).
In this way, the simultaneous equations are solved for all beam combinations, and the terminal reduction method is applied to each simultaneous equation to find the case where the number of user connections is the largest, thereby maximizing the simultaneous user connection rate. An optimal beam combination and base station transmit power can be determined. This method is called the brute force method of antenna beam selection.

  According to the brute force method of antenna beam selection, it is possible to determine the optimum beam combination and base station transmission power that maximizes the user simultaneous connection rate most accurately. It becomes. Therefore, another embodiment of the present invention in which the amount of signal processing is reduced will be described. In the beam selection method of this embodiment, a plurality of, for example, k (1 ≦ k <M) beams with a strong received signal strength at the terminal are extracted, and an optimum beam is used for a beam with a weak received signal strength. Since there is a low possibility, the signal processing amount is reduced as compared with the round-robin method described above by removing it from the search target.

FIG. 5 is a diagram for explaining a case where only the top k beams with strong terminal reception strength are targeted.
Assume that each base station has M beams having different directivity directions as shown in FIG. In this case, when the number of base stations is N, a combination of the beam in all base station becomes as M ^N. Therefore, as shown in (b), k (1 ≦ k <M) beams are selected in descending order of the signal strength received by the terminals belonging to the base station, and a combination of the selected k beams is targeted. The above-described calculation is performed. In the example shown in the figure, the case where the signal strength received by the terminal is strong in the order of beam 3, beam 4 and beam 2 is shown, and only these beams are selected as the calculation target. Thereby, as shown in (c), the number of beam combinations in all base stations to be calculated becomes k ^N ways, and the calculation amount can be greatly reduced. For example, when k = 1, that is, when a beam with the highest received power from the base station is selected in the terminal, the amount of calculation can be reduced most.
Here, the number of beams to be selected in each base station is k, but a different number of antenna beams may be selected for each base station.

The results of evaluating the characteristics of the multi-base station cooperative transmission power control and antenna beam selection control method of the present invention by computer simulation will be described. FIG. 6 is a diagram illustrating a result of evaluating a user simultaneous connection rate, which is a probability that a terminal in the system satisfies the required transmission quality γ _{req, i} . Here, 19 omnicells are regularly arranged in a hexagonal lattice shape, and the propagation path gain is inversely proportional to the distance of 3.5. In the case of an omni antenna, the cell radius, the maximum transmission power of the base station, and the noise power are set so that the reception SNR at the cell edge is 10 dB at the maximum. In the case of a multi-beam antenna, normalization was performed so that the sum of transmission power in all directions was equal to that of the omni antenna. The number of beams per base station of the multi-beam antenna is M = 3, and the directing direction of each beam is shifted by 360 degrees / M = 120 degrees. In addition, the half width of each beam is 120 degrees, and the side lobe level is constant at -15 dB. The propagation path gain and noise power are ideally determined, and the required received SINR (γ _{req, i} ) is assumed to be equal for all users i.

In the figure, a is a case where multiple base station cooperative transmission power control is performed, the above-described Lowest-SINR removal is adopted as a terminal reduction method, and an antenna beam selection control of k = 2 is performed. When multiple base station transmission power control employing Lowest-SINR removal is performed and antenna beam selection control of k = 1 is performed, c performs transmission power control for each base station without performing multiple base station power control. When no antenna beam selection control is performed, d indicates the required reception SINR (γ) when neither the multiple base station power control nor the transmission power control for each base station is performed and the antenna beam selection control is not performed. _{req, i} ) indicates the simultaneous user connection rate.
From this figure, it can be seen that by performing base station cooperative power control and multi-beam selection control, the user simultaneous connection rate can be significantly improved. In addition, when designing a system with a 90% user simultaneous connection rate, by applying antenna beam selection control with k = 2, the SINR of the user is about 1 compared with the antenna beam selection control with k = 1. It can be seen that it can be improved by 5 dB.

1 ₁ to 1 _N : base station, 2 ₁ to 2 _N : terminal, 3: control station

Claims (5)

A multi-base station cooperative transmission power control and antenna beam selection control method for controlling transmission power and beam directions of transmission antennas in the plurality of base stations in a cellular communication system having a plurality of base stations and a plurality of terminals,
Formulating simultaneous equations with the beam direction and transmission power of the transmitting antennas at the plurality of base stations as variables so that the transmission quality of the terminals belonging to the plurality of base stations becomes the required transmission quality, A first step of finding a solution;
A second step of determining, when a solution of the simultaneous equations satisfies a constraint condition of base station power, as a beam direction and a transmission power of a transmission antenna in the plurality of base stations;
When the solution of the simultaneous equations does not satisfy the constraint condition of the base station power, the number of terminals belonging to the selected base station among the plurality of base stations is reduced, and the process proceeds to the first step. A plurality of base station cooperative transmission power control and antenna beam selection control methods.   The multiple base station cooperative transmission according to claim 1, wherein the first to third steps are executed for all combinations of beam directions of transmission antennas possessed by each of the plurality of base stations. Power control and antenna beam selection control method.   In each of the plurality of base stations, a beam direction having the strongest signal strength received by a terminal belonging to the base station is selected as the beam direction of the transmitting antenna of the base station, and the first to third steps The method according to claim 1, wherein the steps are executed.   In each base station of the plurality of base stations, the first to third steps are executed for all combinations of a plurality of beam directions selected in descending order of signal strength received by terminals belonging to the base station. The multi-base station cooperative transmission power control and antenna beam selection control method according to claim 1. A multi-base station cooperative transmission power control and antenna beam selection control device for controlling transmission power and beam direction of a transmission antenna in the plurality of base stations in a cellular communication system having a plurality of base stations and a plurality of terminals,
Formulating simultaneous equations with the beam direction and transmission power of the transmitting antennas at the plurality of base stations as variables so that the transmission quality of the terminals belonging to the plurality of base stations becomes the required transmission quality, A first means for finding a solution;
A second means for determining, when a solution of the simultaneous equations satisfies a constraint condition of base station power, as a beam direction and a transmission power of a transmission antenna in the plurality of base stations;
If the solution of the simultaneous equations does not satisfy the base station power constraint condition, the number of terminals belonging to the selected base station among the plurality of base stations is reduced, and the processing of the first means is executed. A plurality of base station cooperative transmission power control and antenna beam selection control devices.

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